This invention relates to ferroelectric materials which are amorphous instead of crystalline. In particular, it concerns a technique for preparing thin films of amorphous ferroelectric material by condensation from a liquid solution containing precursor compounds.
It has been well established that many crystalline mixed oxide compositions such as barium titanate, BaTiO.sub.3, LiNbO.sub.3, Pb(Zr,Ti)O.sub.3, (Sr,Ba)Nb.sub.2 O.sub.6, and the like, are ferroelectric in nature and both single crystal and polycrystalline forms of these materials have large numbers of practical and potential applications in electronic, opto-electronic, nonlinear optical and piezo-electric devices. Such applications include, for example, optical wave guides, electro-acoustic transducers, high frequency surface acoustic wave devices, pyroelectric infrared detectors, ferroelectric memory cells, ferroelectric photoconductor displays, optical modulators, field effect transistors, metal/insulator/semiconductor transistors and the like.
For many years, it has been believed by the scientific community that ferroelectricity can only exist in a crystalline material with long range order. In 1977, a theoretical discussion suggested that the presence of ferroelectricity in an amorphous glass was not excluded on theoretical grounds and a model for a possible amorphous ferroelectric material was proposed. "Microscopic model for a ferroelectric glass" by M. E. Lines, Physical Review B, 15 (Jan. 1, 1977).
Since then, there have been sporadic efforts to produce an amorphous ferroelectric material without apparent success. Techniques for producing amorphous materials have included RF sputtering or in at least one case by extremely rapid quench freezing of molten lithium niobate and lithium tantalate. "Anomalous dielectric behavior and reversible pyroelectricity in roller-quenched LiNbO.sub.3 and LiTaO.sub.3 glass", by A. M. Glass, M. E. Lines, K. Nassau and J. W. Shiever, Applied Physics Letters, 31 (Aug. 15, 1977). Tantalizing hints of ferroelectricity such as anomalies in the dielectric constant have been noted in amorphous materials. In addition, Glass, et al. noted a pyroelectric response. However, they concluded that "these observations are consistent with ferroelectric behavior, but not conclusive . . . "
To unambiguously show ferroelectricity in a material, it is generally regarded that the most significant indication of ferroelectricity is the well known P-E hysteresis loop. The polarization P as a function of the electric field E shows a characteristic hysteresis loop in an alternating field due to the field required to reverse polarization. Another important criterion is the presence of pyroelectric current, namely current flow from a poled material as temperature is changed.
Ferroelectric materials also have a ferroelectric to paraelectric phase transition temperature T.sub.c sometimes referred to as the Curie temperature. A material may be ferroelectric below the Curie temperature and it loses ferroelectricity abruptly at this temperature. One may also observe ferroelectric domains in the material and a dielectric anomaly is likely to be observed. This anomaly may take the form of three or four orders of magnitude increase in dielectric constant near the Curie temperature.
Some of these phenomena may not be observed in a given sample. More than one of these phenomena should be observed to unambiguously determine that there is ferroelectricity. Observation of a P-E hysteresis loop is regarded as proof of ferroelectricity, although it is still desireable to confirm this by observing pyroelectric current and other ferroelectric phenomenon.
Existing metal oxide-based ferroelectrics fall into two general categories, single crystals and polycrystalline ceramics. Single crystals are typically grown from melts at high temperatures by slowly cooling down certain regions of the melt and allowing the growth of a single crystal. A Czochralski technique may be used. Polycrystalline ceramics may be made through solid state reactions of powders or from a melt. Thin films of ferroelectric material may be made in the form of single crystals or in a polycrystalline form. These thin films have principally been obtained by vapor-phase deposition and sputtering, followed by heating to fully crystallize the deposited film.
It has now been discovered that stable ferroelectricity can be produced in amorphous materials formed by a modified sol-gel technique. The ferroelectric effect has not only been found in mixed metal oxides known to be ferroelectric in their crystalline state, but also in certain single metal oxides never previously known to be ferroelectric. The materials are stabilized, for example, by heating, so that stable ferroelectric properties persist during use of the films.
In recent years, the so called sol-gel technique has been used for preparing crystalline metal oxide-based ferroelectric materials in either thin film or powder form. This technique employs organometallic compounds or metal alkoxides to make a homogenous solution. The solution is typically hydrolyzed to produce a gel which may be precipitated, dried and crushed to form a powder, or the solution may be used for coating a thin film on a substrate. The powders or thin films are then heated above a crystallization temperature to produce a polycrystalline ferroelectric material.
Research has been directed to the deposition and crystallization parameters of the technique on the premise that to obtain ferroelectricity full crystallization is necessary. The morphology of the polycrystalline film has been a major concern since it in large part dictates the characteristics of the film. Since there are shortcomings due to grain boundaries in polycrystalline thin films, efforts have also been directed toward growing single crystal films by the sol-gel technique.
It is desirable to have a technique for forming a ferroelectric material which can be processed at low temperatures so that there is greater freedom in selection of materials compatible with processing of the ferroelectric material. It is desirable that the technique for producing the ferroelectric material be suitable for forming thin films for use in modern electronic and optical devices.